The general structures of computer systems include one or more processing units connected to memory devices and input/output devices, wherein the memory device stores program instructions that are executed by the processing unit to perform the desired functions and the processing unit can transfer data to and from devices through buses.
The evolution history of computer buses includes ISA structures of '80s; and PCI structure, AGP structure, and PCI-X (PCI-extended) structures of '90s, and these structures cannot meet requirements of mass data transfer of peripheral cards nowadays. To solve the problem of insufficient bandwidth, PCI-SIG (PCI Special Interest Group) introduced the new standard for PC I/O: PCI EXPRESS® a trademark of PCI-SIG Corporation ((PCI-E), formerly 3GIO or 3rd Generation I/O) in 2002, and it will gradually replace the present structures.
In the transition from the old structure to the new structure, the originally used PCI-X buses will sooner or later be upgraded to PCI-E buses. In the beginning of the transition period, some PCI-X slots may remain in the boards. However, it is likely that there will be no PCI-X slots after the PCI-E structure totally replaces the PCI-X structure. For low-end computers with low costs (e.g., personal computers), it would be convenient and better to replace the old computers with new computers. For non low-end computers, the connected peripheral devices are relatively expensive (for example, a special storage system including 40 hard disk drives), and it will be problematic to directly replace the old computers with the new computers. For example, in a situation wherein the new computers have only PCI-E slots and the expensive peripheral devices only use PCI-X buses, the PCI-E slots of the new computers and the PCI-X buses of the peripheral devices cannot be connected if the old computers of the PCI-X structure are to be replaced by the new computers of the PCI-E structure. Alternatively, if some specific expensive peripheral devices with PCI-X buses are to be replaced by the peripheral device with PCI-E buses, the cost thereof will be largely increased.
Therefore, there is an urgent need for a mixed use of different bus structures of computers systems and peripheral devices during the transition period from the PCI-X structure to the PCI-E structure. The computer systems can be directly upgraded from the PCI-X structure to PCI-E structure, and the originally used expensive peripheral devices of the PCI-X structure can still be used.